1. Field of the Invention
This invention relates to a numerical controlling unit that controls acceleration of a servomotor in such a manner that the servomotor is not subjected to an overload.
2. Description of the Related Art
There are known various types of numerical controlling units that control acceleration of a servomotor. In controlling the acceleration of a servomotor, an overload of the servomotor may be a problem.
An overload of a servomotor is explained concerning a machine tool by way of an example. A main factor of an overload of the servomotor in the machine tool is thought to be a load by friction in feeding a work piece or tool or by a cutting force and a load by acceleration-deceleration torque for feeding a work piece or tool.
In prior art, in order to prevent an overload of the servomotor in the machine tool, the load by friction in feeding a work piece or tool or by a cutting force is monitored. Then, the speed of the servomotor is limited in such a manner that the load doesn""t cause any overload of the servomotor.
In addition, in order for the load by acceleration-deceleration torque for feeding a work piece not to cause an overload of the servomotor, an upper limit for acceleration is set so as not to cause any overload even if accelerating and/or decelerating is frequent.
In the above prior art, the upper limit for acceleration for feeding a work piece is set at a value on the assumption that accelerating and/or decelerating is frequent. This means that performance in feeding the work piece can not be used sufficiently.
Therefore, the object of this invention is to provide a numerical controlling unit that can control acceleration of a servomotor in such a manner that the servomotor is not subjected to an overload, and that can use performance thereof in feeding the work piece sufficiently.
To achieve the above object, this invention is characterized by following features. That is, this invention is a numerical controlling unit comprising: a instructing part that provides a command acceleration for a servomotor; a parameter storing part that stores a threshold parameter about a load of the servomotor; a load measuring part that measures the load of the servomotor; a load comparing part that compares the load measured by the load measuring part with the threshold parameter; an acceleration adjusting part that changes the command acceleration for the servomotor based on a result compared by the load comparing part; and a controlling part that controls the servomotor based on the changed command acceleration.
According to the feature, the command acceleration for the servomotor is suitably changed based on the result of comparison between the load measured by the load measuring part and the threshold parameter. Thus, it can be prevented that the servomotor is subjected to an overload while using the maximum performance in feeding a work piece.
Preferably, when the load measured by the load measuring part is over the threshold parameter, the acceleration adjusting part is adapted to decrease the command acceleration for the servomotor by a value corresponding to a differential between the load measured by the load measuring part and the threshold parameter.
According to the feature, the load of the servomotor is suitably limited to the threshold parameter as its maximum. Thus, it can be prevented more surely that the servomotor is subjected to an overload.
In the case, it is preferable that: when the acceleration has been decreased once and then the load measured by the load measuring part becomes lower than the threshold parameter by a predetermined value or more, the acceleration adjusting part is adapted to increase the command acceleration for the servomotor by a value corresponding to a differential between the load measured by the load measuring part and the threshold parameter, within a range of the previous decrease of the acceleration. Through such a suitable control by the acceleration adjusting part, the performance in feeding a work piece can be used to the maximum.
For example, the load measuring part may be adapted to measure the load of the servomotor from a command torque for the servomotor.
In addition, this invention is a numerical controlling unit comprising: a instructing part that provides a command acceleration for a servomotor; a parameter storing part that stores a threshold parameter about a load of the servomotor; a load estimating part that estimates the load of the servomotor; a load comparing part that compares the load estimated by the load estimating part with the threshold parameter; an acceleration adjusting part that changes the command acceleration for the servomotor based on a result compared by the load comparing part; and a controlling part that controls the servomotor based on the changed command acceleration.
According to the feature, the command acceleration for the servomotor is suitably changed based on the result of comparison between the load estimated by the load estimating part and the threshold parameter. Thus, it can be prevented that the servomotor is subjected to an overload while using the maximum performance in feeding a work piece.
Preferably, when the load estimated by the load estimating part is over the threshold parameter, the acceleration adjusting part is adapted to decrease the command acceleration for the servomotor by a value corresponding to a differential between the load estimated by the load estimating part and the threshold parameter.
According to the feature, the load of the servomotor is suitably limited to the threshold parameter as its maximum. Thus, it can be prevented more surely that the servomotor is subjected to an overload.
In the case, it is preferable that: when the acceleration has been decreased once and then the load estimated by the load estimating part becomes lower than the threshold parameter by a predetermined value or more, the acceleration adjusting part is adapted to increase the command acceleration for the servomotor by a value corresponding to a differential between the load estimated by the load estimating part and the threshold parameter, within a range of the previous decrease of the acceleration. Through such a suitable control by the acceleration adjusting part, the performance in feeding a work piece can be used to the maximum.
For example, the load estimating part may be adapted to read a subsequent plurality of blocks of a control program for the servomotor, and then estimate the load of the servomotor from an effective value of an acceleration-deceleration torque by the control program.